The AcceptMutex directives sets the
method that Apache uses to serialize multiple children accepting
requests on network sockets. Prior to Apache 2.0, the method was
selectable only at compile time. The optimal method to use is
highly architecture and platform dependent. For further details,
see the performance tuning
documentation.

If this directive is set to Default, then the
compile-time selected default will be used. Other possible
methods are listed below. Note that not all methods are
available on all platforms. If a method is specified which is
not available, a message will be written to the error log
listing the available methods.

flock

uses the flock(2) system call to lock the
file defined by the LockFile directive.

fcntl

uses the fcntl(2) system call to lock the
file defined by the LockFile directive.

The BS2000Account directive is available for
BS2000 hosts only. It must be used to define the account number for
the non-privileged apache server user (which was configured using the
User directive). This is
required by the BS2000 POSIX subsystem (to change the underlying BS2000
task environment by performing a sub-LOGON) to prevent CGI scripts
from accessing resources of the privileged account which started the
server, usually SYSROOT.

This controls the directory to which Apache attempts to
switch before dumping core. The default is in the
ServerRoot directory, however
since this should not be writable by the user the server runs
as, core dumps won't normally get written. If you want a core
dump for debugging, you can use this directive to place it in a
different location.

Core Dumps on Linux

If Apache starts as root and switches to another user, the
Linux kernel disables core dumps even if the directory is
writable for the process. Apache (2.0.46 and later) reenables core dumps
on Linux 2.4 and beyond, but only if you explicitly configure a CoreDumpDirectory.

For safety reasons this directive is only available if the server was
configured with the --enable-exception-hook option. It
enables a hook that allows external modules to plug in and do something
after a child crashed.

There are already two modules, mod_whatkilledus and
mod_backtrace that make use of this hook. Please have a
look at Jeff Trawick's EnableExceptionHook site for more information about these.

The Group directive sets the group under
which the server will answer requests. In order to use this
directive, the server must be run initially as root. If
you start the server as a non-root user, it will fail to change to the
specified group, and will instead continue to run as the group of the
original user. Unix-group is one of:

A group name

Refers to the given group by name.

# followed by a group number.

Refers to a group by its number.

Example

Group www-group

It is recommended that you set up a new group specifically for
running the server. Some admins use user nobody,
but this is not always possible or desirable.

Security

Don't set Group (or User) to root unless
you know exactly what you are doing, and what the dangers are.

The Listen directive instructs Apache to
listen to only specific IP addresses or ports; by default it
responds to requests on all IP interfaces. Listen
is now a required directive. If it is not in the config file, the
server will fail to start. This is a change from previous versions
of Apache.

The Listen directive tells the server to
accept incoming requests on the specified port or address-and-port
combination. If only a port number is specified, the server listens to
the given port on all interfaces. If an IP address is given as well
as a port, the server will listen on the given port and
interface.

Multiple Listen directives may be used to
specify a number of addresses and ports to listen to. The server will
respond to requests from any of the listed addresses and ports.

For example, to make the server accept connections on both
port 80 and port 8000, use:

Listen 80
Listen 8000

To make the server accept connections on two specified
interfaces and port numbers, use

Listen 192.170.2.1:80
Listen 192.170.2.5:8000

IPv6 addresses must be surrounded in square brackets, as in the
following example:

Listen [fe80::a00:20ff:fea7:ccea]:80

Error condition

Multiple Listen directives for the same ip
address and port will result in an Address already in use
error message.

The maximum length of the queue of pending connections.
Generally no tuning is needed or desired, however on some
systems it is desirable to increase this when under a TCP SYN
flood attack. See the backlog parameter to the
listen(2) system call.

This will often be limited to a smaller number by the
operating system. This varies from OS to OS. Also note that
many OSes do not use exactly what is specified as the backlog,
but use a number based on (but normally larger than) what is
set.

The LockFile directive sets the path to
the lockfile used when Apache is used with an AcceptMutex value of either
fcntl or flock. This directive should
normally be left at its default value. The main reason for changing
it is if the logs directory is NFS mounted, since
the lockfile must be stored on a local disk. The PID
of the main server process is automatically appended to the
filename.

Security

It is best to avoid putting this file in a world writable
directory such as /var/tmp because someone could create
a denial of service attack and prevent the server from starting by
creating a lockfile with the same name as the one the server will try
to create.

The MaxClients directive sets the limit
on the number of simultaneous requests that will be served. Any
connection attempts over the MaxClients
limit will normally be queued, up to a number based on the
ListenBacklog
directive. Once a child process is freed at the end of a different
request, the connection will then be serviced.

For non-threaded servers (i.e., prefork),
MaxClients translates into the maximum
number of child processes that will be launched to serve requests.
The default value is 256; to increase it, you must also raise
ServerLimit.

For threaded and hybrid servers (e.g.beos
or worker) MaxClients restricts
the total number of threads that will be available to serve clients.
The default value for beos is 50. For
hybrid MPMs the default value is 16 (ServerLimit) multiplied by the value of
25 (ThreadsPerChild). Therefore, to increase MaxClients to a value that requires more than 16 processes,
you must also raise ServerLimit.

The MaxMemFree directive sets the
maximum number of free Kbytes that the main allocator is allowed
to hold without calling free(). When not set, or when set
to zero, the threshold will be set to unlimited.

The MaxRequestsPerChild directive sets
the limit on the number of requests that an individual child
server process will handle. After
MaxRequestsPerChild requests, the child
process will die. If MaxRequestsPerChild is
0, then the process will never expire.

Maximum number of idle threads. Different MPMs deal with this
directive differently.

For perchild the default is
MaxSpareThreads 10. This MPM monitors the number of
idle threads on a per-child basis. If there are too many idle
threads in that child, the server will begin to kill threads
within that child.

For worker, leader and threadpool the default is MaxSpareThreads 250.
These MPMs deal with idle threads on a server-wide basis. If there
are too many idle threads in the server then child processes are
killed until the number of idle threads is less than this number.

For mpm_netware the default is
MaxSpareThreads 100. Since this MPM runs a
single-process, the spare thread count is also server-wide.

Minimum number of idle threads to handle request spikes.
Different MPMs deal with this directive
differently.

perchild uses a default of
MinSpareThreads 5 and monitors the number of idle
threads on a per-child basis. If there aren't enough idle threads
in that child, the server will begin to create new threads within
that child. Thus, if you set NumServers to 10 and a MinSpareThreads value of 5, you'll have
at least 50 idle threads on your system.

worker, leader and
threadpool use a default of MinSpareThreads
75 and deal with idle threads on a server-wide basis. If
there aren't enough idle threads in the server then child
processes are created until the number of idle threads is greater
than number.

mpm_netware uses a default of
MinSpareThreads 10 and, since it is a single-process
MPM, tracks this on a server-wide bases.

The PidFile directive sets the file to
which the server records the process id of the daemon. If the
filename is not absolute then it is assumed to be relative to the
ServerRoot.

Example

PidFile /var/run/apache.pid

It is often useful to be able to send the server a signal,
so that it closes and then re-opens its ErrorLog and TransferLog, and
re-reads its configuration files. This is done by sending a
SIGHUP (kill -1) signal to the process id listed in the
PidFile.

The PidFile is subject to the same
warnings about log file placement and security.

Note

As of Apache 2 it is recommended to use only the apachectl script for (re-)starting or stopping the server.

Apache uses a scoreboard to communicate between its parent
and child processes. Some architectures require a file to facilitate
this communication. If the file is left unspecified, Apache first
attempts to create the scoreboard entirely in memory (using anonymous
shared memory) and, failing that, will attempt to create the file on
disk (using file-based shared memory). Specifying this directive causes
Apache to always create the file on the disk.

Example

ScoreBoardFile /var/run/apache_status

File-based shared memory is useful for third-party applications
that require direct access to the scoreboard.

If you use a ScoreBoardFile then
you may see improved speed by placing it on a RAM disk. But be
careful that you heed the same warnings about log file placement
and security.

The server will set the TCP buffer size to the number of bytes
specified. Very useful to increase past standard OS defaults on
high speed high latency (i.e., 100ms or so, such as
transcontinental fast pipes).

For the prefork MPM, this directive sets the
maximum configured value for MaxClients for the lifetime of the
Apache process. For the worker MPM, this directive
in combination with ThreadLimit sets
the maximum configured value for MaxClients for the lifetime of the
Apache process. Any attempts to change this directive during a
restart will be ignored, but MaxClients can be modified during
a restart.

Special care must be taken when using this directive. If
ServerLimit is set to a value much higher
than necessary, extra, unused shared memory will be allocated. If
both ServerLimit and MaxClients are set to values
higher than the system can handle, Apache may not start or the
system may become unstable.

With the prefork MPM, use this directive only
if you need to set MaxClients higher than 256 (default).
Do not set the value of this directive any higher than what you
might want to set MaxClients to.

The StartServers directive sets the
number of child server processes created on startup. As the number
of processes is dynamically controlled depending on the load,
there is usually little reason to adjust this parameter.

This directive sets the maximum configured value for ThreadsPerChild for the lifetime
of the Apache process. Any attempts to change this directive
during a restart will be ignored, but ThreadsPerChild can be modified
during a restart up to the value of this directive.

Special care must be taken when using this directive. If
ThreadLimit is set to a value much higher
than ThreadsPerChild,
extra unused shared memory will be allocated. If both
ThreadLimit and ThreadsPerChild are set to values
higher than the system can handle, Apache may not start or the
system may become unstable. Do not set the value of this directive
any higher than your greatest predicted setting of ThreadsPerChild for the
current run of Apache.

The default value for ThreadLimit is
1920 when used with mpm_winnt and
64 when used with the others.

Note

There is a hard limit of ThreadLimit 20000 (or
ThreadLimit 15000 with mpm_winnt)
compiled into the server. This is intended to avoid nasty effects
caused by typos.

This directive sets the number of threads created by each
child process. The child creates these threads at startup and
never creates more. If using an MPM like mpm_winnt,
where there is only one child process, this number should be high
enough to handle the entire load of the server. If using an MPM
like worker, where there are multiple child processes,
the total number of threads should be high enough to handle
the common load on the server.

The default value for ThreadsPerChild is
64 when used with mpm_winnt and
25 when used with the others.

The User directive sets the user ID as
which the server will answer requests. In order to use this
directive, the server must be run initially as root.
If you start the server as a non-root user, it will fail to change
to the lesser privileged user, and will instead continue to run as
that original user. If you do start the server as root,
then it is normal for the parent process to remain running as root.
Unix-userid is one of:

A username

Refers to the given user by name.

# followed by a user number.

Refers to a user by its number.

The user should have no privileges that result in it being
able to access files that are not intended to be visible to the
outside world, and similarly, the user should not be able to
execute code that is not meant for HTTP requests. It is
recommended that you set up a new user and group specifically for
running the server. Some admins use user nobody, but
this is not always desirable, since the nobody user
can have other uses on the system.

Security

Don't set User (or Group) to root unless
you know exactly what you are doing, and what the dangers are.

With the perchild MPM, which is intended to
server virtual hosts run under different user IDs, the
User directive defines the user ID for the
main server and the fallback for <VirtualHost> sections without an
AssignUserID directive.